In pilot studies we have noted hyperaggregable platelets and accelerated clot formation in the aftermath of acute stroke and transient ischemic attacks (TIA). However, the importance of these abnormalities in th pathogenesis of clinical disease is unkown. We propose to study two groups of patients, one with completed strokes and the other with TIA, immediately after the ictus and serially over a three year period to examine two aspects of hemostasis in cerebrovascular disease: First, we will investigate a possible causal relationship between diaschisis (acute, transient, bilateral reduction of cerebral blood flow in patients with unilateral cerebral infarction) and disorders of platelets and clotting. We have previously observed a temporal relationship. Second, we have hypothesized that persistent abnormalities of hemostasis after a stroke imply a primary role for platelets or clotting factors in the pathogenesis of the clinical disorders under investigation. Consequently, patients with persistent abnormalities will be at greater risk of stroke recurrence (acute stroke group) or initial stroke (TIA group) than those patients whose hemostatic functions are only transiently abnormal.